I'm torn about this one. I really don't like watching the game through a screen, but I get around this by sitting in the upper deck. I wouldn't take a kid under about 10 to sit anywhere shy of typical outfielder depth in the lower bowl, so maybe that's an argument in favor of netting. I doubt it's practical to age-restrict those sections.
I also agree with the posters who point out that the narrowing of foul territory in newer parks is a real problem here. It may make sense for the rule to be generic, something like "Any field-level section for which the closest seat is within X feet of home plate must be protected by netting." That way you wouldn't end up putting netting in front of sections in Oakland that are only marginally closer to home plate there than they are to San Francisco.